1. Field of the Invention
This invention relates to laminated window assemblies, particularly of the type used in aircraft. More particularly, this invention relates to a method for overcoming laminate failure in polycarbonate-polyvinyl acetal laminated windows.
2. Brief Description of the Prior Art
It is well known in the art to make laminated windows with glass and rigid polycarbonate sheeting. Polycarbonate is optically clear, has a refractive index approximating that of glass and has excellent impact and load-bearing properties. However, polycarbonate does not laminate well to glass. The rigid polycarbonate sheet and the glass have such different coefficients of thermal expansion that a laminate prepared from the two will crack on cooling from the high temperatures needed for lamination. When lower laminating temperatures are used, the bond strength between the glass and the polycarbonate is insufficient for commercial applications.
It has been proposed in the art that the glass can be bonded to polycarbonate indirectly through the use of a thermoplastic interlayer material which could absorb the stresses of high temperature and pressure lamination and yet still provide strong bonding for the glass and for the polycarbonate. Thus, Saunders in U.S. Pat. No. 3,388,032 and Moynihan in U.S. Pat. No. 3,458,388 disclose the use of polyurethane interlayers to bond polycarbonate to glass.
McCombie in U.S. Pat. No. 3,388,034 discloses the use of polyvinyl butyral sheeting as an interlayer material for bonding glass to polycarbonate. For the polyvinyl butyral material to be an effective interlayer material, that is, in contributing good impact properties, it is necessary that it be plasticized. However, the conventional plasticizer for polyvinyl butyral, for example, triethylene glycol di-(2-ethyl butyrate), is unsatisfactory for use with polycarbonate. The polycarbonate develops a haze after a short period of time in contact with polyvinyl butyral so plasticized, making the resultant window unsightly looking and unsatisfactory for aircraft use. Further, the laminate bond strength between the polycarbonate and the polyvinyl butyral is not as strong as desired and as a result, the laminate containing polyvinyl butyral bonded to polycarbonate does not have optimum impact and load-bearing properties.
Buckley in U.S. Pat. No. 3,388,033 recognized this problem with conventionally plasticized polyvinyl butyral and suggested a specially plasticized polyvinyl butyral, that is, one plasticized with a phosphate plasticizer such as tricresyl phosphate. Although the special plasticizer prevents hazing, the bond strength developed between plasticized polyvinyl acetal and polycarbonate under normal laminating conditions is at the most only about 5 pounds per lineal inch. This low degree of bonding is inadequate and can result in delamination, which may have serious consequences. For example, delamination between the two plies may cause an opaqueness in the window which obscures vision. Also, delamination reduces the impact and load-bearing properties of the window, making it more susceptible to failure from bird or projectile impacts.
It would therefore be desirable to overcome the source of failure in laminated windows containing adjacent plies of plasticized polyvinyl acetal and polycarbonate. It would be desirable to provide multi-layer laminated articles containing adjacent plies of polycarbonate and polyvinyl acetal which could be used as transparent windows for such applications as military and commercial aircraft.
This invention provides such laminated articles and also provides a method for overcoming the source of failure in such laminates.